Magnetically tuned high frequency circuits



Nov. 21, 1939. R. 1 HARVEY MAGNETICALLY IUNED HIGH FREQUENC'Y CIRCUITSOriginal Filed Deo. 3l, 1935 2 Sheets-Sheet l H r vey TORNEYS.

INVENTOR.

NOV. 21, 1939. R HARVEY 2,180,413

MAGNETICALLY TUNED HIGH FREQUENCY CIRCUITS Original Filed Dec. 31, 19352 Sheets-Sheet 2 jj@ l l A7 0 THE/17 CoA/ a 7 COKE l IN1/EN TOR.

Hubert L Ha 11e y TORNEYS.

Patented Nov. 21, 1939 UNITED STATES MAGNETICALLY TUNED HIGH FREQUENCYCIRCUITS Robert L. Harvey, Oaklyn, N. J., assignor to Radio Corporationof America, a corporation of Del-1,

aware Original application December 3-1, 1935, Serial No. 56,993.Divided and this application November 30, 1936, Serial No. 113,468

17 Claims.

My invention relates to high frequency communicat-ion tuning systems,and more particularly to adjustable magnetically tuned resonant circuitsand inductors for operation at radio frequencies. This is a divisionalof application S. N. 56,993, 'filed December 3l, 1935. Heretofore it hasbeen the practice in superheterodyne radio receivers to employ, in theintermediate frequency amplifier, a fixed inductor, air core type oftransformer with a pair of semi-adjustable screw type condensers withmica dielectric, mounted on an insulated base, side by side, for tuningthe primary and secondary coils respectively of said transformer. Such astructure is shown in De Tar application No. 621,002, filed July 6,1932. Trouble has been experienced, after the receivers have beenassembled and in use, with changes in frequency adjustment. Because ofthe fact that intermediate frequency transformers must be preciselytuned to a givenxed frequency, a slight change in the capacity of thecondensers results in detuning of the frequency to an extent thatnecessitates realignment by a service man. Notwithstanding carefulprecautions in design, it appears to have been impossible to preventsuch changes occurring in the tuning caused by Warping of the plates,ageing of the dielectric, temperature and moisture changes, etc.

In the tuned radio frequency amplifiers, by way of additional example,it has been customary to accurately match up inductances of the coils inlcascaded stages, tunable over a band of frequencies by means of aplurality of variable condensers ganged for single control, inaccordance with the teachings of Giblin #1,842,937 patent. Various meanshave been used to vary the inductance of one or more of the coils in thematching process in production, one means in general use being thesliding of coil turns as disclosed in the De Tar Patent 1,860,176. Anobjection to the latter means has been that there is no way toconveniently vary the position of the turns, which have been cemented inplace, e. g., by means of a simple tool, as in the case of trimmercondensers. Although this method has proven highly useful, a delicatehand operation is necessary. To provide structure for a screw drivertype of adjustment, as by means of a variometer arrangement would entailconsiderable increase in expense without sufficient compensatingadvantages.

For many years it has been known that paramagnetic material operativelydisposed in the field of an inductor used in radio frequency workproduces certain desirable electrical advantages,

provided the material is in'such form and arrangement as to minimizeenergy consuming eddy current and hysteresis losses.

The most satisfactory cores for coils heretofore used in high frequencycircuits have consisted of extremely nely divided or comminuted magneticmaterial, such as iron dust, held together with a suitable insulatingbinder. As an example of extreme fmeness of subdivision, it has beenconsidered necessary to use pure iron powder small enough to passthrough a 300 mesh screen for use with inductors adapted for thebroadcast frequency range of 550-1500 kilocycles. The cost of productionof such finely divided material, as by chemical reduction of iron oxide,has been expensive, as well as the production of the finished moldedcore. It has been necessary, because of the low resistance of the iron,to provide that the particles shall be well insulated electrically fromeach other to reduce eddy current losses; in some cases the particles ofpure iron, or alloy, have been oxidized, and in other cases aninsulating powder has been mixed with the iron powder to minimizeelectrical contact among the particles.

The problem of manufacturing satisfactory cores for operation at radiofrequencies is quite different from that of cores for audio frequencyWork. I am aware that iron oxide was proposed many years ago for loadingcoils in telephony, see Lee and Colpitts #705,935, but in recent yearsworkers skilled in the art have apparently considered it necessary to goto the trouble and expense of providing pure iron or alloys and haveactually taken oxide of iron and reduced it by various processes to pureiron. Whereas Lee and Colpitts disclosed that ferroso-ferric oxide(Fea04) was suitable for loading coils, they apparently considered itnecessary to go to the trouble of producing it synthetically.Superficial tests with the ore magnetite would lead one skilled in theart to assume that the material is unsatisfactory, but afterconsiderable research I have discovered ways and means wherebymagnetite, inexpensive and rather plentiful, may be employedsuccessfully for the purposes disclosed without changing chemically theform of the ore. For electrical reasons, as well, I prefer the naturalmagnetite,

It is, accordingly, an object of my invention to provide a novel andimproved inductor tuned coupling unit for use in radio systems toimprove the gain and selectivity of such systems, and factor of merit ofcircuits therein.

It is a further object of my invention to pro- REISSUED MAY 26 1942 videan improved magneticaliy tunable transformer or inductor for use in highfrequency communication systems, which, by reason of its novel design,is substantially lower in cost and smaller in size, without a sacrificein gain and selectivity relative to apparatus heretofore in general use.

A still further object of my invention is to provide an`improvedresonant coupling unit, avoiding the necessity for adjustable trimmercondensers, for use in the intermediate frequency amplifier of asuperheterodyne radio receiver, which, by reason of its novel design, issubstantially more stable in regard to the fixed frequency adjustmentthroughout the useful life of the receiver, notwithstanding changes inhumidity, temperature, and age.

A still further object of my invention is to provide an improvedmagnetic core, and method of making same, which will have a highpermeability and low core loss when disposed in a varying magnetic fieldset up by radio frequency currents.

More specifically, it is an object of my invention to provide animproved resonant coupling unit and radio chassis assembly, which, byreason of my novel design, is adapted to low cost production and ischaracterized by convenient tuning adjustment of the coupling unit inthe assembly.

A still further object of my invention is to provide a radio frequencytransformer with one or more adjustable permeable core structures forchanging the self inductance of one or more of the transformer coilswithout materially changing the coupling coeiiicient between coils.

In accordance with my invention, the primary and secondary coils of anintermediate frequency transformer of a superheterodyne receiving systemare shunted respectively by small fixed capacitors, although, in somecases, the capacitors are not used, and the coils are adjustably tunedto the desired intermediate frequency by moving molded cores of powderedmagnetic material, preferably comminuted magnetite, in the fieldsrespectively of the coils. By reason of my novel design, the transformercoils and the entire shielded coupling unit can be made substantiallysmaller than air core transformers for the same requirement as to factorof merit, or they can be made larger with a substantial improvement inthe factor of merit. The result of using magnetite cores is to reducethe combined cost of the coils and capacitors more than the cost isincreased by the addition of the iron cores.

Another advantage is the elimination of adjustable trimmer condensers,the electrical constants of which change considerably with humidity,temperature and aging.

A still further improvement is in lower loss tuned circuits (higher Q)which results in higher gain and greater selectivity per amplifierstage, which can be converted into still lower cost and smaller size byreducing the size of the shielding container until the circuit lossesare the same as for the standard air core design.

Another advantage is that the weight of the coupling unit issubstantially reduced, a reduction of 60% having occurred in certainunits made in accordance with my invention. This is particularlyadvantageous for radio apparatus used on aircraft.

It is believed that the invention will be better understood from thefollowing detailed description of certain embodiments of myv inventionand from the accompanying drawings in which Fig. 1 is a full sized sideelevational view of a portion of radio chassis having mounted thereonradio apparatus of a superheterodyne system, made in accordance with myinvention;

Fig. 2 is a side elevational View in section, drawn to scale, of anintermediate frequency transformer or coupling unit shown in Fig. 1;

Figs. 3 and 4 are schematic circuit diagrams of portions of asuperheterodyne radio system that will serve to illustrate certainapplications of my new and improved coupling unit shown in Fig. 2;

Fig. 5 is a side elevational view,'partly in section, of a modified formof my radio frequency coupling unit;

Fig, 6 is a circuit diagram of an antenna input circuit for a radioreceiving system that will serve to illustrate one of the uses of thecoupling unit shown in Fig. 5;

Fig. 7 is an enlarged cross sectional view of an element of the couplingunit shown in Fig. 2;

Fig. 8 is an enlarged plan view of the element shown in section in Fig.7

Fig. 9 is a modified and preferred form of an intermediate frequencytransformer or coupling unit made in accordance with my invention;

Fig. l0 is a bottom view of the transformer unit of Fig. 9 together witha cut-way section of a chassis on which the unit is mounted;

Fig. 1l represents characteristic curves of my improved apparatus;

Fig. lla is an enlarged view, partly in section, of the coil and corestructure of the unit of Figs. 2 and 9 shown in functional relation tothe curves of Fig. 11;

Fig. l2 is a View, in side elevation, of a slightly modied form of myinvention, corresponding to the coil and core structures of Figs. 2 and9;

Fig. 13 is a circuit diagram of a still further modification of myinvention corresponding to Fig. 12 and illustrating an ideal developmentthereof; and

Fig. 14 is a view, in side elevation, partly in section, of the moldingdevice used in forming the molded magnetic material in accordance withmy invention.

It will be understood, however, that these embodiments of my inventionare merely illustrative and that the invention is not limited to theseforms.

Referring to the drawings, an intermediate frequency coupling unit,housed in a shielding container I is mounted on the metal chassis 3 of asuperheterodyne radio set, preferably with the unit extending through anopening 5 in the chassis. For the conservation of space, the lowerportion of the unit extends below the surface of the chassis and ispreferably mounted thereon by means of a clamp 'I which encircles thecontainer I and is clamped thereto by means of a bolt and nut 9, Theclamp is provided with a pair of horizontal flange portions II which areriveted to the chassis at I3 in a manner that will appear obvious. Theunit may readily be removed for servicing by disconnecting the outerends of leads I4, I5, IB and loosening bolt 9. The coupling unit isprovided with tuning adjustment screws I'I and I9 at its respectiveends, below and above the surface of the chassis respectively. Thisprovides a very convenient and accurate adjusting means, without the useof special tools, for purposes of convenient assembly as well assubsequent servicing, if necessary.

For the reduction of capacity coupling, one of the leads 2I from theunit is brought out from the top and is provided with a clip 23 adaptedto engage the grid terminal of a vacuum tube 25,

one of the new metal envelope small sized vacuum tubes (Radiotron type#6K7) being shown. By reason of my novel design, it will be noted thatthe size of the coupling unit, particularly that portion extending abovethe chassis, compares favorably with the size of the small vacuum tube.`

By way of example, the dimensions ofthe unit are 311g', long by 1% indiameter', as compared to 5 x 1%" for the usual air core transformerdesign. The weight of the new unit is 0.1 lb. as against 0.25 lb. forthe air core design.

Referring more in detail to the interior of the coupling unit, as shownin Fig. 2, the 4transformer consists of universal sectional woundprimary and secondary coils 21 and 29 xedly mounted on a sleeve 3I ofinsulating material such as fiber tubing. The tubing is concentricallymounted within the shield container I by means of the end closing plates33 and 35 respectively. The end plates are provided with eyelets 31 fromwhich depend terminals 39 which in turn support and electrically connectcondensers 4I and 43 respectively. The terminal leads oi' the coils 21and 29 are also electrically attached to these terminals 39.

Referring more in detail to Figs. 7 and 8, the end plates each consistof a pair of discs 41 and 49, preferably of laminated synthetic phenolresin material, or any other suitable insulating, with a sheet of rubber5I cemented therebetween. This unit is assembled by applying adhesivematerialbetween adjacent surfaces and by applying pressure. For thepurpose of retaining the ends of the tubular sleeve 3|, one of the platesections 49 is provided with a circular slot 53 into which an end of thesleeve 3I is adapted to snugly fit. The function of the rubber sheet 5Iis to frictionally grip the adjusting screws I1 and I9 at the threadedbore 51 for insuring against lost motion. The bores 51 in the end platesare threaded, as by a self tapping action by the screws I1 and I9respectively. The rubber sheet also frictionally grips the coil tubingand prevents turning while adjusting the core.

Referring back to Fig. 2, the transformer coils 21 and 29 are providedwith adjustable cores 59 and 6I, respectively, disposed in slidingrelation within the bore of the tube 3I. The cores consist preferably ofgranular magnetite ore. This material, as sifted natural sized sandparticles or as granules crushed from larger sized pieces of ore, ismixed with an insulating binder, preferably phenol condensation resin,known as Bakelite, cold-molded under moderate pressure, and cured undermoderate temperature, the adjusting screws I1 and I9 respectively beingmounted in the ends of the cylindrical cores 59 and 6I respectively.

Referring again to Fig. 2, I have shown an inner-lining 65 for theshielding container, preferably of the same molded magnetic material asin the cores. This innerliner may be molded in sections, two sectionsbeing shown in the drawings and assembled into the container, andseparated therefrom by a thin sheet of insulating material 61, ifdesired. The insulation may be omitted and the innerliner cemented inthe container. The effect of the molded shielding innerliners is toeffectively reduce the reactive effect of the shielding container uponthe coils. This advantage can be used to improve the efficiency orfactor of merit of the coils with a given construction, or can be usedto make possible a substantially smaller overall construction ofcoupling unit, with the same efficiency as before.

Further, by way of example, the following specifications are given of anintermediate frequency coupling transformer made in accordance with n.yinvention and adapted to operate at 460 kc., although the data is basedon a design omitting the shielding innerliner 61, as in Fig. 9, laterdescribed. The coils 21 and 29 are each wound with single silk enamelfive strand #40 litz wire in four sections on a ten mil ber tube 3|,inside diameter and 3" long. Each coil is wound in four sections, ithaving been found that this number gives better practical design thanthree or five sections, for reducing capacity and for obtaining otheradvantages hereinafter described. There are 75 turns of wire per coilsection, in eight layers, wound 1/8" wide with spacing between sections,and 1/2" between inner sections, respectively, of primary and secondary.The iron cores are 237,4 in diameter and 1%" long with a 9&2" brassscrew I1, 3A," long inserted 1%" into the end of the cores. The coilinductances, without magnetite core, and the fixed capacities should beheld to present day production tolerances of i 5%. The cores can be heldto an effective inductance tolerance of i 1%. With these tolerancesprovision is desirably made for adjusting the cores to give aninductance change of each coil of around plus and minus 15%.

The cores in the drawings are shown in about the normal intendedoperating position. It is desirable that the circuit constants be madesuch that it is not necessary to insert the cores into the supportsleeve 3I a distance farther than about mid-way between the innermostadjacent two sections of each of the transformer windings respectively.I have found, in accordance with my invention, that the cores may beadjusted t0 this position, which I will call the maximum position, tovary the self inductance of each coil without materially changing thecoefcient of coupling between primary and secondary. This coupling islargely determined by the inner adjacent sections, respectively, ofprimary and secondary coils, discussed more fully in connection withFig. 11a. In order to insure against travel of the cores beyond thispoint, various limiting structures may be employed if desirable,although I have shown an inner sleeve 13 of reduced diameter securedwithin the sleeve 3| as by any suitable cementing material. Screws I1and I9 may, if desired, be reduced in length and amount that will limitthe travel to the maximum position.

The reason for the avoidance of variable coupling between primary andsecondary with different positions of the cores, is that productiondiiiiculties are avoided. It is intended that movement of the cores varyonly the self inductances, respectively, in order to adjust the resonantpoint of the tuned circuits. The coupling is determined mainly by therelative spacing of primary and secondary coils on the support sleeve 3|under given conditions of design of the surrounding shield container.For production this coupling is readily predetermined and it is desiredthat it be a fixed quantity. It can be seen that any change inindividual units of this coupling, and resulting change in selectivity,would quite seriously upset this uniformity of production. In otherwords, the cores should never extend into the two adjacent primary andsecondary coil sections which have the greatest mutual inductancebetween them.

Curve L in Fig. l1 shows the relation of core position with respect toeither one of the coils of Fig. 2 and the resulting self inductance.Coil 21, enlarged in form, is chosen by way of example, in associatedFig. 11. Figs. 11 and 11a are drawn to line up so that the corepositions plotted as abscissa in Fig. 11 correspond to the physicalrelations in Fig. 11a. With the 3A" core in the maximum position, theinductance is shown to be 1380 micro-henries, whereas with the coreremoved, the inductance is found to be 530 microhenries. The effectivepermeability of the core in maximum position is about 2.6. With a oneinch length core, the effective permeability was about 3.

As indicated in Fig. 11a, the core should not extend to the left furtherthan the position marked A, the working range being chosen betweenpoints A and B. One reason for this will be seen by referred to curvesD, E and F, plotted between percent coupling between primary andsecondary, and the positions of each core, the different curvesrepresenting different spacings between primary and secondary. Theprimary and secondary cores were shifted equally for each reading. CurveD represents a spacing between innermost sections of respective coils ofcurve E a spacing of l, and curve F a spacing of The measurements weretaken at a frequency of 1000 cycles.

The curves show how critical the spacing is. Curve D shows that a quiteflat coupling curve for the working range may be obtained with a spacingof In the structure of Fig. 2, however, a spacing of 1/2 was used, thecurve E indieating that the results were satisfactory. Curve F showsthat with close spacing between innermost sections of the transformerwindings, the percent coupling varied to a greater degree, and to anundesirable degree in the region to the left of the working range.

Allowing for liberal production tolerances of the various elements, thecore positions for resonance will lie between points A and B, thedesired working range of the iron core, a core travel of about 13g"`with the average position at C. In other words, the end of the core isadapted to move between opposite ends of the next to the innermost coilsection. This adjustment will give an inductance of about 1060microhenries (plus and minus 15%), and an average effective permeabilityof two. Referring to Curve Q, it has been found that in the same L/Cratio, the Q or factor of merit, and consequently the gain andselectivity, does not increase appreciably as the core is moved inwardlybeyond the mid-point of the coil (point B) the Q rises fromseventy-eight, core out, to eighty-nine, with core halfway in the coil(point B), and to only ninety-one with iron core in the maximumeffective permeability position (point A). This may be accounted for byincreased distributed capacity and losses due to the presence of thecore. The core and shield material employed in the above examples, is aniron oxide ore sometimes called lodestone, and consists chemically ofone part FeO and two parts FezOs. The natural ore is ground, and/orsifted, to the required granular size and magnetically separated fromsilica and other foreign material. According to Dictionary `of AppliedChemistry," Thorpe, vol. 13, 1912 edition, page 378, the followingdefinitions are given:

Magnetite, or Magnetic Iron-Ore.-A mineral of the spinel group,consisting of magnetic oxide of iron, Fe3O4 or FeOFezOa; an importantore of iron (Fe 72.4 p. c.). Sharply developed crystals with brightfaces are not uncommon; these belong to the cubic system and usuallyhave the form of the regular octahedron or the rhombicdodecahedron.Granular to compact masses are, however, more abundant. The colour isironblack with a dull, submetallic lustre and a black streak. Sp. gr.5.18; hardness 6. The mineral may be always readily recognized by itsstrong magnetic character; small fragments are picked up by a magnetisedknife-blade. Only occasionally are specimens magnetic with polarity (v.loadstone). As small grains and crystals, magnetite is of widedistribution in many kinds of igneous rocks, especially the darkercoloured with a low silica percentage. In such rocks it sometimes formsrich segregations available for mining; as in the Ural Mountains and atKirunavara and Gellivara in Swedish Lapland. Other important deposits,e. g., some of those in southern Sweden and Norway, have been formed bythe metamorphism of pre-existing iron-ores, where they have beensubjected to the baking action of intrusive masses of igneous rock.Extensive deposits of magnetite are also mined in the crystal lineArchaean rocks of the Adirondack region of New York and in Canada.Inorganic and Therostical Chemistry" by Mellor, vol. 8, part 2, page732. magnetite exhibits a wide variation in composition, for theextremes in 30 analyses were:

By commercial analysis I have found that the magnetite giving bestresults comes from the Adirondacks and is the ideal compound as listedin Mellor` and described as FesO-4 or FeO, FezOs (Fe 72.4%) in Thorpe.It is noted that the reference Fe 72.4% in Thorpe (which is thepercentage of iron by atomic weight) is the same as the combined ironpercentage in Mellor, i. e., 24.11+48.29=72.4%. The ratio of Fe as inFeO and Fe as in FezOs is 1:2. The ratio by Weight of FeO to FezOa isabout 31:69.

M agnetite particles size-The required neness of magnetite particles isdetermined by: First, desired permeability and permissible loss (the nerparticle cores have lower loss but lower permeability); secondly,mechanical strength and appearance (ner particles will make stronger andsmoother surface cores). As between permeability and losses, there is anoptimum compromise for a certain frequency range. The larger particlesgive higher permeability because a given mass of ore material is morecompact under the conditions as formed in nature than when particles arepresent and molded with a binder synthetically.

For frequencies around 460 kilocycles, I nd that iron particles passing40 mesh and held back on 60 mesh screens make the best compromise cores.The iron particle size is not very critical over fairly wide ranges ofoperating frequencies. Thus, cores designed for 460 kc. may also be usedfor 175 kc., although 30-40 mesh appears to be a somewhat bettercompromise. Likewise, 60-80 mesh is somewhat better than 40-60 mesh for1000 kc., and above. A neness' of 325 mesh, or smaller, is desirable forultra high frequencies of the order of 10,000 kc.

Bz`ndeT.-The binder serves to hold the iron particles together, also asan insulator between magnetic particles, and probably as a lubricant toallow the particles to slide closely together during molding. It shouldbe noted that the same material is used as binder, insulator, andlubricant. For this I prefer to use Bakelite, a resinous phenolcondensation product, preferably starting with uncured Bakelite inpowder form and adding a solvent. A synthane Bakelite varnish in liquidform may, if desired, be used. The proportion of binder and iron varieswith size of iron particles and molding process. I have' found that amixture in the ratio of one part by volume of binder to fourteen partsof 40-60 mesh magnetite makes satisfactory cores, according to myinvention.

Molding procesa- The preferred molding process consists of: (a) Mixingfourteen parts magnetic particles and one part dry bakelite powder in amill. (b) Adding about four parts of the above mixture, by volume to onepart of a liquid solvent such as acetone. As the mixing process iscontinued, part of the binder solvent will evaporate, and the mass willin time break up and return to a granular mixture, leaving a dry coatingof Bakelite insulation on the iron oxide particles; (c) Pouring thecoated magnetite (like sand) into the hopper of the mold and applyingabout three tons pressure to the mold for cold molding (Fig. 14); (d)Removing the cores, cold molded, and placing in an oven, and curing thecores at 150 C. to 200 C. for about two hours.

The correct pressure used in molding magnetite is an important andcritical factor in the production of a core having satisfactorycharacteristics for radio frequency work, as are some of the otherfactors involved in my process, as will be seen from the following:Whereas it is desirable to employ a very high pressure in molding inorder to increase the density of the magnetic material in order tocorrespondingly increase the permeability, too great a pressure causesthe magnetic particles to break through the insulation material withwhich it is mixed, resulting in increased eddy current loss. rFoo low apressure, and resulting lower density of magnetic particles, produces apermeability that is too low for satisfactory results. I have found,however, that because of the relatively high electrical specificresistance of magnetite as compared to pure iron, a much greater amountof points of electrical contact are allowable between particles withoutunduly increasing losses. This means that a greater density of materialmay be used.

If two much binding material is used the excess binder displacesmagnetite particles and results in a loss of permeability. On the otherhand if too small an amount is used there results a poor insulationbetween magnetic particles and increases losses.

If the mesh size is too small, there is not enough magnetic material ina given volume, and there is a resulting lowering of permeability.Nature has compressed a maximum amount of magnetic material in a givenpiece and it appears diilcult to duplicate this synthetically bypressing together many fine particles. It is. therefore, desirable, fromthe point of view of permeability, to employ natural particles as largeas possible, compromising with eddy current losses. If the particles aretoo large the eddy current losses increase for well known reason.

Referring to Fig. 14, I have found that a better core is made bythe useof a double and pressure mold. A mold sleeve 95 forms with a baseplunger 96 a hopper into which the magnetic mixture is poured forforming the core 59 with the screw insert I1. An upper plunger 91,having a recess to accommodate the screw I1 ts snugly into the upper endof sleeve 95. The sleeve flts snugly around the plungers and is free tomove with respect to both plun'gers as the pressure is applied to theupper end of plunger 91, thereby resulting in a compressed core ofuniform density throughout. The inner lining for the shield is made inthe same manner as described for the core, with suitable changes made inthe mold.

'I'he adjusting screw may be molded in the cores initially, or the coresmay be drilled out and the screw inserted, and cemented with a drop ofcollodion or liquid Bakelite after one hour of heat treatment, followedby a second heat treatment of one hour.

Referring to Fig. 3, I have shown diagrammatically the circuit of myimproved intermediate frequency coupling unit with its primary coil 21connected in the plate circuit of a first detector thermionic tube 15andwith its secondary 29 connected in the input grid of an intermediatefrequency amplifier tube 11. In circuits of this nature, it is desirablethatcapacity coupling between primary and secondary be at a minimum. Ithas been found that high capacity coupling disturbs the characteristicresponse curve in some amplifiers. The coils are coupled preferably withoptimum coupling. Whereas, in transformer units, heretofore used forcircuits of this type, there existed a substantial amount of undesirablecoupling between the trimmer condensers mounted side by side on thetransformer base. The condensers 4I- and 43 are remotely mounted atopposite ends of the unit as shown in Fig. 2 and are, in addition, ofsubstantially smaller size by reason of their being of the xed,nonadjustable, type.

Referring to Fig. 4, I have shown diagrammatically an application of myimproved coupling unit operatively disposed between the lastintermediate frequency amplifier 19 and a diode detector 8|. It has beenwell known for some time that, in using an intermediate diode detectorand/or automatic volume control rectifier, with inherent capacitycoupling between primary and secondary circuits, that a reduction in thesize of the by-pass capacitor 83 in the secondary circuit across thediode load resistor to improve audio iidelity, it ris impossible toobtain a satisfactory symmetrical resonance response curve. Thiscondition is madestill more unsatisfactory if the diode transformersecondary 29 is tapped, as shown, for the purpose of obtaining greaterselectivity by reducing the load on the transformer. I have found that,by reason of my improved construction wherein the condensers 4| and 43of Fig. 2 are small in dimension and are mounted at remote ends of theunit, the inherent undesired capacity is substantially reduced.Furthermore, bringing the leads out at opposite ends in the mannershown, also reduces the capacity as do other novel features of thedesign. The result is that a symmetrical resonance response curve isobtained in using my device in the circuit of Fig. 4 even when capacitor83 is much smaller than used in previous tainer and cement it in place,as by collodion. In either case the container and the magnetite comprisecontiguous layers of a composite shield. This construction is providedwith end support plates 33 and 35 as in the case of Fig. 2, althoughonly one adjustable core of magnetic material is employed. The plate 35is held in spaced relation by means of a spacer 81 of insulatingmaterial, the ends of the shield container being bent over at 34 tosecure the parts in place. The coil 89 may represent a single inductoror a primary and secondary respectively of a broadly tuned radiofrequency or intermediate frequency transformer wherein the distributedcapacity of the coils is used instead of physical condensers as part ofthe tuned circuit. In some arrangements it is desirable that thesecondary be Wound over the primary coil to insure maintaining constantcoupling regardless of movement of the core. In such a case, the innerlayers of coil 89 constitute the primary and the outer circumferentiallayers the secondary. A structure of this nature may be employed as adiode driving transformer in the circuit of Fig. 4, the condenser 43being omitted, if desired, from the secondary circuit.

Fig. 6 represents diagrammatically an antenna input circuit wherein theunit in Fig. 5 may be used to advantage. In this case the core isemployed to give a line adjustment of the inductance of the secondary 89in order to match its inductance with that of the inductances ofsucceeding stages adapted to be tuned to the same frequency. It may alsobe employed to obtain the desired inductance adjustment of coil 89 inrelation to the inductance of a superheterodyne oscillator, the tuningcondenser of the oscillator being ganged with the tuning condenser 9| inthe antenna circuit. Such an arrangement obviates the necessity ofadjusting turns, the usual practice as explained in the rst part of thespecification, besides giving a very substantial increase in the gain ofthe antenna input circuit due to the lower resistance of the circuit.This arrangement has been found to be particularly useful forapplication to automobile radio receivers where it is desirable toobtain maximum gain in the antenna circuit.

Referring to Figs. 9 and 10, a modified form of my invention as shown inFig. 2, the shield conis made square, in section, with rounded corners.The upper end has the edges bent over at |0| to form a flange againstwhich the top insulating support plate |03 abuts. The lower end of theshield is provided with a pair of bolts |05, riveted to the container at|06, for securing a bottom insulating plate |01 in place as well as theentire inside coil assembly, with the aid of nuts |09. The plate |01lies within the walls of can |00 except for outwardly extending ears |08which engage cut away sections of the shield as an abutment. The free.ends of the bolts, extending beyond the nuts |09, are adapted toprotrude through apertures in a radio chassis ||3 for securing the unitin place on the chassis, with the aid of nuts in registry with anopening I5 therein for access to terminals and adjusting screw.

A threaded bushing ||2, as of brass, is mounted centrally in an openingin each support plate |03 and |01 and is secured in place as by astaking or upsetting operation at ||1. The bushings serve to carry theadjusting screws |1 and 9 for the cores. To prevent binding of the coresin the coil support sleeve 3|, in case of slight inaccuracies ofalignment, the threaded portion of the bushing is limited in length andis remote from the sleeve 3 thereby permitting a little side play. Tightfrictional action between the screw |9, for example, and the bushingthreads is obtained by means of an angular spring ||9 which engages thescrew I9 through a slot |2| in the bushing exposing the screw, the backof the bushing, thereby forcing the screw against the threads in oneside of the bushing.'

The bushings also serve to support the sleeve 3| with a force t at itsends over shoulders |23, respectively of the bushings. The shoulders arelongitudinally knurled at |24 for purposes of giving a tight lit and astrip of paper or fiber |25 is disposed around the sleeve 3| at each endto reinforce it where it is forced on over the shoulders |23.

Terminals |21 are disposed in special apertures |29 in each of the endsupport plates, and are held in place by deforming laterally theprojections |3|. 'I'he terminals are adapted to have soldered to theirinwardly extending portions leads from the inside coils 21 and 29 (Fig.2) certain of them carry the condensers 4| and 43 as in Fig. 2. Externalleads |35 are soldered to the external ends of the terminals. A cap |33is disposed over the upper end of the shield |00 for minimizing capacitycoupling to the otherwise exposed terminals on the top of the insulationplate |03.

Referring to Fig. 12, while for production it is desirable to haveuniform spacing between coil sections, the inner adjacent sections 28and 30 of primary and secondary coils 21 and 29 may be spaced a greateramount from their other associated coil sections in order to furtherincrease the independence of adjustment of the self inductances withrespect to coupling which it is value. This permits of a greaterinsertion of the core into the remaining coil sections without exceedingthe permissible effect upon coupling between primary and secondary. Itis desirable that the adjacent coil sections 28 and 30 be at the lowradio frequency potential ends of the primary and secondary,respectively, to reduce capacity coupling. The same is true of Fig. 2.

A further desirable modication of my invention is illustrated in Fig.12. A coupling coil 34 of a few number of turns, is wound between thefirst and second sections, at the high radio frequency potential end ofthe primary coil. For this position it can be seen that the mutualcoupling between the primary coil 21 and the coupling coil 34, willremain unchanged while the molded core is moved over the working rangeas shown in Fig. 11a. In accordance with the teachings of Carlson Patent1,871,405, a switch means 36 is provided for throwing the coil 34 in orout of the circuit to control broadness of the resonance curve of thetransformer circuit. A suitable value of resistance` 38 is provided inseries with the coupling coil, as in the Carlson patent, to preventdouble peaked response curve.

Carrying this feature further to obtain a maximum independence ofinductance adjustment by cores 59 and 6| with respect to the coupling, Ihave shown in Fig, 13, coupling coil sections 28 and 30 magneticallyisolated from the remaining sections. They are coupled together, and arepreferably provided with an auxiliary core 60 which may be adjustable.The coupling between circuits is determined mainly by sections 28 and 30which may be isolated by placing them in a separate shield, or moreinexpensively, in

the same container but mounted at right angles as indicated.

I claim as my invention:

1. In a radio frequency transformer comprising an elongated shieldhousing, and magnetic means therefor, a pair of coupled inductors insaid housing having adjusting members extending beyond the opposite endsrespectively of said housing for individual inductance adjustment ofsaid inductors, said adjusting members having substantially no effect onthe coupling between said inductors.

2. In a resonant high frequency coupling unit, an inductor, a core ofmolded magnetic granular material, a shield housing closely disposedaround said inductor, means carried by said housing for supporting saidcore for adjustment within said inductor, said housing comprising anouter sheath of sheet metal and an inner layer of molded paramagneticmaterial interposed between said inductor and sheath and characterizedby low eddy current losses, whereby the factor of merit of said inductoris increased over what it would be in the absence of said inner layerand said core.

3. The invention as set forth in claim 2 characterized in that saidinner layer of magnetic material is molded in close physical andelectrical relation with said metal sheath.

4. A radio frequency coupling device comprising a plurality of inductivereactance elements including adjustable cores of comminuted magneticmaterial, an elongated electrical shield housing surrounding saidelements, a plurality of circuit conductors, including an input and anoutput terminal of high radio frequency potential and at least one lowradio frequency potental terminal for connecting said elements withradio apparatus external to said shield, said input and output terminalsbeing disposed at opposite ends respectively of said shield housing forreduction of capacity coupling.

5. The invention as set forth in claim 4 further characterized by thefact that means for adjusting the positions of said cores with respectto said reactance elements are provided at each end of said housing.

6. A radio frequency coupling device comprising an elongated shieldhousing, support means carried at opposite ends of said housing, atubular sleeve coaxially suspended in said housing between said supportmeans, a reactor disposed around said sleeve, a core of magneticmaterial disposed within said sleeve and arranged for movement to varythe inductance of said reactor, a bushing carried by one of said supportmeans, said bushing being provided with means for adjusting the positionof said core and with structure for engaging an end of said sleeve insupporting relation therewith.

7. The invention as set forth in claim 6 further characterized by thefact that a similar bushing is carried by the other of said supportmeans, and said support structure on said bushing consists of an annularshoulder for engaging the ends of said sleeve.

8. The invention as set forth in claim. 2 further characterized in thatsaid inner layer of magnetic material is insulated electrically fromsaid metal sheath.

9. In a radio frequency coupling device, a tuning inductor, a shieldhousing surroundingI said inductor, said housing comprising an outersheath of sheet metal, an inner layer of comminuted paramagneticmaterial, and means for electrically insulating said sheath from saidlayer of magnetic material.

10. The invention as set forth in claim 9 further characterized by thefact that said inductor is provided with an adjustable magnetic core,said core and said sheath of magnetic material being of a highresistance magnetic oxide.

11. A coil system for radio apparatus comprising an inductor, acomposite metal shield surrounding said inductor, and an inner shield ofcomminuted paramagnetic material disposed between said inductor and saidmetal shield and relatively closer spaced to the latter over asubstantial area, whereby the factor of merit of said inductor isincreased over what it would be in the absence of said second namedshield.

12. The invention as set forthin claim 11 characterized in that saidinner shield is of high specific resistance relative to said metalshield.

13. The invention as set forth in claim 11 characterized in that saidinner shield comprises a magnetic oxide of iron.

14. In a radio frequency coupling device, a tuning inductor, a compositeshield housing surrounding said inductor, said housing comprising anouter sheath of sheet metal, an inner layer of comminuted paramagneticmaterial, said sheath and said layer being connected together in closephysical and electrical relation over a substantial area.

15. In a high frequency coupling device, an inductor, a compositeelectrical shield disposed in magnetic relation to said inductor, saidshield comprising a layer of sheet metal on the side remote from saidinductor and a contiguous layer of comminuted paramagnetic material onthe side of said shield adjacent to but relatively spaced substantiallyfrom said inductor.

16. A coil system for radio apparatus comprising an inductor, alaminated shield adjacent said inductor and comprising a layer of sheetmetal and a relatively high resistance layer of comminuted paramagneticmaterial disposed between said inductor and said sheet metal layer,whereby the factor of merit of said inductor is inl creased over what itwould be in the absence of said second named layer.

17. In a high frequency coupling device, a composite shield comprising alayer of comminuted paramagnetic material characterized by highresistance to current flow and being highly effective as anelectromagnetic shield, and a relatively low resistance metal layerdisposed in contiguous relation .therewith over a substantial area, saidmetal layer being grounded for effective electrostatic shielding.

ROBERT L. HARVEY.

